The present invention relates to silicon-on-sapphire transducers, and more particularly to an improved method for making the same.
As is known, as a deflecting diaphragm, single-crystal sapphire has certain unique advantages. Not only is it a single crystal that displays no mechanical hysteresis when deflected, exhibiting only elastic deformation, but is ultra-resistant to almost any chemical attack or etching. While this may be an advantage in a finished transducer, it causes significant difficulties in fabrication.
Sapphire has a modulus of about 70xc3x97106 PSI as compared to that of silicon which, in the transverse direction in the surface plane  less than 100 greater than  (as defined on FIG. 4A), is about 20xc3x97106 PSI. As a result, for the same electrical output from a piezoresistive Wheatstone bridge silicon grown or otherwise fastened to sapphire, the flexing diaphragm must be thinner by the ratio of                                                         YaL              2                        ⁢                          O              3                                Ysi                                    (        1        )            
This means that to fabricate a pressure transducer of sapphire with the same surface stress as one of silicon, since the surface stress is proportional to                               1          Y                ⁢                              a            2                                t            2                                              (        2        )            
where a is the radius of the deflecting portion and t is the thickness, a diaphragm of sapphire must have a much larger diameter than one of silicon, or be much thinner.
However, because of the inert nature of sapphire, it is almost impossible to thin the sapphire diaphragm by conventional means. Further, making a diaphragm of sufficient size to get enough stress lowers the number of sensors that can be made from an individual slice as well as lowering the resultant natural frequency of the finished sensors. Additionally, slices of commercially available sapphire are usually thick (about 0.020xe2x80x3). However, for a relatively small diameter sensor, a thickness on the order of 0.005 inches is required.
The present invention is designed to overcome these constraints and to produce a relatively smaller silicon on sapphire sensor with enhanced characteristics.
An improved method for making silicon-on-sapphire transducers including the steps of: sputtering or otherwise growing a silicon layer on a first surface of a commercially available first sapphire wafer.
Affixing a second sapphire wafer including a series of apertures to the first surface the first sapphire wafer, preferably by means of E.S. bonding, fusion bonding or any other suitable means as is understood by those possessing ordinary skill in the pertinent art.
Lapping and polishing a second surface of the first sapphire wafer until the first sapphire wafer is reduced to a desired thickness. The second sapphire wafer, which was previously secured to the first wafer, serves to strengthen the first wafer during this process.
Growing and/or sputtering a silicon layer on the now polished surface of the first sapphire wafer. This silicon layer is then oxidized using any well known, conventional technique.
A third wafer containing a series of sensor networks is formed using any suitable process. The third wafer includes p+ areas which include a sensor network and a group of contact areas extending from the sensor network. Outside of the contact areas, another p+ area separate from the contact areas but surrounding them, is also formed. The third wafer is then preferably fusion-bonded to the oxidized surface of the first sapphire wafer, and the non p+ areas are removed preferably by using a conductivity selective etch.
Appropriate areas in the p+ contact regions are preferably metalized using a high temperature metal system such as platinum silicide, titanium, or platinum.
A fourth wafer made of glass or any suitable, non-conductive material, is prepared such that there are a series of apertures corresponding to the various metalized contact areas on the first wafer. Additionally, there is preferably provided a slight depression in the fourth wafer corresponding to the area of the sensor network to allow the diaphragm to deflect without touching the glass.
Finally, the glass wafer is preferably sealed using an ES Bond to the p+ contact areas, and the apertures over the metalized areas may be filled with a glass metal frit to make electrical contacts to metalized areas of the p+ regions. If the seal of the glass wafer to the p+ contact area is not through a central aperture in the glass wafer over the deflecting portion of the first sapphire wafer, an absolute sensor will result. If there is a through-central aperture, a gage or differential sensor will result.